A Level Chemistry Revision "Relative Atomic Mass"

Summary

This video teaches the concepts of relative isotopic mass and relative atomic mass, explaining how they are calculated using isotope data. It highlights that relative isotopic mass is a whole number without units, and relative atomic mass is a weighted mean mass also without units, derived from the abundance of isotopes.

Takeaways

• 🔬 Atoms have extremely small masses, making their direct measurement challenging.
• 📌 Scientists use relative mass to compare atomic masses, with carbon-12 as the reference point.
• 🌐 Carbon-12 is assigned a mass of exactly 12, and other atoms' masses are measured relative to 1/12 of its mass.
• 🧪 Isotopes are atoms of the same element with different numbers of neutrons, resulting in different masses.
• 🏗️ Relative isotopic mass is the mass of an isotope compared to 1/12 the mass of carbon-12.
• 📊 Relative isotopic mass is always a whole number and has no units.
• 📚 The periodic table lists the relative atomic mass of elements, which is calculated from the relative isotopic masses.
• 📈 Relative atomic mass is the weighted mean mass of an element's isotopes, taking into account their abundance.
• 🧩 The relative atomic mass is not always a whole number, reflecting the average mass based on isotopic abundance.
• 📘 To calculate relative atomic mass, use the equation involving the relative isotopic masses and the percent abundance of each isotope.

Q & A

What is the main topic of the video?

The main topic of the video is the concept of relative isotopic mass and relative atomic mass, and how to calculate relative atomic mass from isotope data.

Why are relative masses used in chemistry?

Relative masses are used in chemistry because atoms are extremely small and their actual masses are very tiny, making it impractical to use absolute masses in calculations.

What is the reference isotope for relative mass in chemistry?

The reference isotope for relative mass in chemistry is carbon-12.

How is the mass of carbon-12 defined in terms of relative mass?

In chemistry, the mass of carbon-12 is defined as exactly 12, and 1/12 of its mass is counted as 1 relative mass unit.

What are isotopes and how do they differ?

Isotopes are atoms of the same element with a different number of neutrons, which results in different masses for each isotope.

What is the definition of relative isotopic mass?

Relative isotopic mass is defined as the mass of an atom of an isotope compared with 1/12 the mass of carbon-12.

Why is the relative isotopic mass always a whole number?

The relative isotopic mass is always a whole number because it is a ratio comparing the mass of an isotope to a fraction of the mass of carbon-12, which is a fixed reference.

What is the formula used to calculate the relative atomic mass of an element?

The formula to calculate the relative atomic mass is the sum of the products of the relative isotopic masses and their respective percent abundances, divided by 100.

Why are relative atomic masses not whole numbers on the periodic table?

Relative atomic masses are not whole numbers on the periodic table because they are weighted means based on the abundance of each isotope, resulting in a non-integer value.

Can you provide an example of calculating the relative atomic mass using the given formula?

Yes, for chlorine with isotopes chlorine-35 (75% abundance) and chlorine-37 (25% abundance), the relative atomic mass is calculated as (35 * 0.75) + (37 * 0.25) = 35.5.

What is the significance of the relative atomic mass in the periodic table?

The relative atomic mass in the periodic table represents the average atomic mass of an element, taking into account the natural abundance of its isotopes, and is used in chemical calculations.

Outlines

00:00

🔬 Understanding Relative Isotopic and Atomic Mass

This paragraph introduces the concept of relative isotopic mass and relative atomic mass, explaining their definitions and importance in chemistry. It emphasizes the small size and mass of atoms and how scientists use carbon-12 as a reference point, assigning it a mass of exactly 12. The paragraph also touches on isotopes, which are atoms of the same element with different numbers of neutrons, and their relative masses. The video script invites viewers to learn the definitions and apply them to calculate the relative atomic mass from isotope data.

📊 Calculating Relative Atomic Mass from Isotope Data

The second paragraph delves into the calculation of relative atomic mass, which is the weighted mean mass of an element's atoms compared to 1/12 of the mass of carbon-12. It explains that this value is derived from the relative isotopic masses of the element's isotopes and their respective abundances. The paragraph provides the formula for calculating relative atomic mass and gives examples with chlorine, thallium, and magnesium isotopes, demonstrating how to apply the formula to find the average atomic mass of these elements.

Keywords

💡Relative Isotopic Mass

Relative isotopic mass is defined as the mass of an atom of an isotope compared to 1/12 the mass of carbon-12. It is a whole number without units and represents the atomic mass of a specific isotope of an element. In the video, it is used to explain how scientists compare the masses of different isotopes, such as the two isotopes of copper with relative masses of 63 and 65.

💡Relative Atomic Mass

Relative atomic mass is the weighted mean mass of an atom of an element compared to 1/12 the mass of carbon-12. It takes into account the abundance of each isotope and is used to calculate the average atomic mass of an element. The video emphasizes that this value is not always a whole number due to the weighted mean calculation, as seen with copper's relative atomic mass of 63.5.

💡Isotopes

Isotopes are atoms of the same element that have different numbers of neutrons, resulting in different masses. The video uses copper as an example, highlighting its two main isotopes with different relative isotopic masses. Isotopes are fundamental to understanding the concept of relative atomic mass.

💡Carbon-12

Carbon-12 is a specific isotope of carbon used as a standard in chemistry for measuring relative masses. The video explains that in chemistry, carbon-12 is assigned a mass of exactly 12, and all other atomic masses are compared relative to it.

💡Abundance

Abundance refers to how common an isotope is within a naturally occurring element. The video uses the abundance of isotopes to calculate the relative atomic mass, such as the 75% abundance of chlorine-35 and 25% abundance of chlorine-37.

💡Atomic Mass

Atomic mass is the mass of an individual atom, typically measured in atomic mass units (amu). The video discusses how atomic mass is used in the context of relative isotopic and atomic masses, particularly when calculating the average mass of an element's atoms.

💡Weighted Mean

A weighted mean is an average that takes into account the relative importance or frequency of data points. In the video, the relative atomic mass is calculated using a weighted mean of the relative isotopic masses, considering the abundance of each isotope.

💡Periodic Table

The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number, electron configuration, and recurring chemical properties. The video mentions the periodic table as a reference for the relative atomic masses of elements.

💡Chlorine-35 and Chlorine-37

Chlorine-35 and chlorine-37 are isotopes of the element chlorine with different relative isotopic masses. The video uses these isotopes to demonstrate the calculation of chlorine's relative atomic mass, with chlorine-35 being more abundant at 75%.

💡Thallium-203 and Thallium-205

Thallium-203 and thallium-205 are isotopes of thallium with different relative isotopic masses and abundances. The video provides an example of calculating thallium's relative atomic mass using the weighted mean of these two isotopes.

💡Magnesium Isotopes

Magnesium has three naturally occurring isotopes: magnesium-24, magnesium-25, and magnesium-26. The video uses these isotopes to illustrate the calculation of magnesium's relative atomic mass, accounting for their respective abundances.

Highlights

Introduction to the concept of relative isotopic mass and relative atomic mass.

Explanation of the challenges in measuring the mass of atoms due to their extremely small size.

Introduction of carbon-12 as the reference standard for relative atomic mass in chemistry.

Definition of relative mass as being relative to carbon-12, with carbon-12 having a mass of exactly 12.

Discussion on isotopes as atoms of the same element with different numbers of neutrons and masses.

Use of copper isotopes as an example to illustrate relative isotopic masses.

Definition of relative isotopic mass as the mass of an isotope compared to 1/12 the mass of carbon-12.

Points to learn about relative isotopic mass: uniqueness to each isotope, whole number values, and unit-less nature.

Introduction of relative atomic mass as the weighted mean mass of an element's atoms compared to carbon-12.

Explanation of how the weighted mean accounts for the abundance of each isotope.

Clarification that relative atomic mass has no units and is typically not a whole number.

Equation for calculating relative atomic mass using the relative isotopic masses and their abundances.

Example calculation of chlorine's relative atomic mass with its two isotopes and their abundances.

Example calculation of thallium's relative atomic mass with its isotopes and abundances.

Final example calculation of magnesium's relative atomic mass considering its three isotopes and their abundances.

Summary of the ability to define and calculate relative atomic mass from isotope data after the video.

Transcripts

I am welcome back to precise lessons by

the end of this video you should be able

to define relative isotopic mass and

relative atomic mass you should then be

able to calculate relative atomic mass

from isotope data now this video is all

about the masses of atoms the problem is

that atoms are extremely smaller so

they've got extremely small masses

here's the mass of one atom of hydrogen

and it concedes a tiny number scientists

solved this problem by using relative

mass masses and chemistry are relative

to carbon-12 which is an isotope of

carbon we call these relative masses

because their relative to carbon-12 in

chemistry we say that atoms of carbon-12

have a mass of exactly 12 and then 1/12

of the mass of this atom is counted as 1

at GCSE we saw that isotopes are atoms

of the same element with a different

number of neutrons and different masses

the good example is copper which has got

two main isotopes atoms of one of the

isotopes have a relative mass of 63 and

atoms of the other isotope have a

relative mass of 65 we call these values

relative isotopic masses relative

isotopic mass is defined as the mass of

an atom of an isotope compared with 1/12

the mass of carbon-12 you need to learn

that definition as you could be asked in

your exam now there are several points

that you need to learn about relative

isotopic mass firstly there's one

relative isotopic mass for each isotope

of an element secondly relative isotopic

mass is always a whole number it's never

a decimal and lastly relative isotopic

mass has got no units coming up welcome

to look at relative atomic mass

[Music]

okay now if you look in your periodic

table you can see that the mass number

for copper is actually 63 point 5 this

is called the relative atomic mass and

we calculate this value from the

relative isotopic masses here's the

definition of relative atomic mass and

again you need to learn this definition

relative atomic mass is the weighted

mean mass of an atom of an element

compared with 1/12 or mass of carbon-12

now the mean is weighted for the

abundance of each isotope in other words

how common an isotope is then again

relative atomic mass has got no unit if

you look at your periodic table you can

see that virtually all of the relative

atomic masses are not whole numbers and

that's because relative atomic mass is a

mean we can't let relative atomic mass

using this equation relative atomic mass

equals the relative isotopic mass of

isotope 1 multiplied by the percent

abundance of isotope 1 added to the

relative isotopic mass of isotope 2

multiplied by the percent abundance of

isotope 2 and then all divided by 100 in

the exam you could be asked to calculate

relative atomic mass given data on

isotopes so let's try some examples

chlorine has two isotopes chlorine 35

with an abundance of 75 percent on

chlorine 37 with an abundance of 25

percent calculate the relative atomic

mass for chlorine so pause the video now

and try this yourself putting the

numbers into the equation it gives us a

relative atomic mass of 35 point 5 for

chlorine here's another example thallium

has two isotopes thallium 2o 3 with an

abundance of 29.5%

and thallium 205 with an abundance of

70.5% calculate the world of atomic mass

of thallium so pause the video now and

try this yourself okay putting the

numbers into the equation it gives us a

relative atomic mass for thallium of 204

point four here's one final example

magnesium has three isotopes magnesium

24 with an abundance of 79 percent

magnesium 25 with an abundance of 10% on

magnesium 26 with an abundance of 11%

calculate the relative atomic mass of

magnesium now as you can see we've

adjusted the equation to take into

count the three isotopes so pause the

video now and try this question yourself

okay putting the numbers into the

equation it gives us a value for the

world of atomic mass of magnesium of

twenty four point three okay so

hopefully now you should be able to

define relative isotopic mass and

relative atomic mass you should then be

able to calculate relative atomic mass

from isotope data

[Music]